Self-aligning, fluid-driven pumping unit

ABSTRACT

A reciprocating well pumping unit includes a fluid-driven cylinder to drive a reciprocating rod assembly. A valve controls fluid delivery to the cylinder, the valve having a valve spool capable of being positioned in a first position or a second position, the valve in the first position causing fluid flow that allows the lifting rod to extend, the valve in the second position causing fluid flow that allows the lifting rod to retract. A detent assembly provides resistance to movement of the valve spool between the second and first positions. A control rod is coupled to and moves with the lifting rod. A spring is capable of storing energy as the control rod is moved in the first direction and a rod stop engages the spring, the spring engaging the valve spool as energy is stored in the spring to urge the valve spool toward the first position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalApplication No. 61/875,561, filed Sep. 9, 2013, which is herebyincorporated by reference.

BACKGROUND

The present disclosure relates to an improved type of oilfield pumpingunit used to reciprocate a down-hole rod pump. Historically,conventional oilfield pumping units used a “walking beam” pivoting on aground-based support frame. The beam is rocked back and forth by a crankarm connected to a rotary gear-drive. The beam is typically connected tothe rod string via a wire rope, and the reciprocating motion of the beamprovides the reciprocal lifting and lowering of the rod string. Thesepumping units were relatively large, massive structures, necessitated bythe overhung lifting loads.

More recently, hydraulically powered pumping units have becomeincreasingly popular. In one configuration, a linear motion systememploys a single hydraulic cylinder centered above or below the polishedrod. Often, a mast is used to suspend a single hydraulic cylinder abovethe polished rod. The polished rod is directly coupled to the cylinder,and hydraulic pressure applied to the ring side of the piston causes thecylinder to retract, thus lifting the rods. The mast can be attached tothe ground, the casing, or the tubing. Other configurations haveemployed an offset cylinder with one or more pulley wheels are attachedto the rod-end of the cylinder. A wire rope passes across the pulleywheel such that one rope end is attached to the rod string and the otherend is dead-lined. As the cylinder extends, the wire rope transmits alifting motion to the rod string.

SUMMARY

Described herein is a compact artificial lift hydraulic pumping systemproviding simplified installation, longer life and ease of maintenance.A beam is pivotally mounted to a wellhead, ground, or other anchorstructure, and one or more hydraulic cylinders are asymmetricallymounted with respect to the beam. The mounting is designed to pivot andslide about the pivot so as to provide self-aligning lift forces to thedown-hole pump rod string. The self-aligning nature of the pump systemeliminates side loads on both the hydraulic cylinder and polished rod.

In some embodiments, the system comprises a beam asymmetrical andparallel to the polished rod. The base of the beam is pivotally orrigidly attached to the wellhead, tubinghead or casing and one or morehydraulic cylinders are mounted parallel but axially offset from thecenter of the pivoting beam. In some embodiments, the end of thehydraulic cylinder is attached to a traveling assembly that comprises atraveling head that rolls along a track formed by the profile of thebeam. The polished rod is attached to the traveling head such that thereciprocating movement of the hydraulic cylinders is thus coupled tocause a similar movement of the rod string. In one embodiment, thetraveling assembly comprises a rotating member, such as a sprocket orsheave, connected to the rod end of the hydraulic cylinder. Thetraveling assembly further includes a flexible linkage, such as a chain,cable, wire, or rope passing over the rotating member and providing alifting force to the polished rod.

Some embodiments require no elevated mast to suspend a hydrauliccylinder above the polished rod. The entire pumping unit may beassembled at ground-level, and then pivotally erected into place. Thiscan be done without the use of overhead lifting equipment such as acrane or boom truck. Some embodiments may be configured to lift the rodswhen hydraulic pressure is applied to cap end of the hydraulic cylinderand the cylinder is extending. In the prior art utilizing a singlecylinder suspended above the well, the ring side of the cylinder must beused to lift the rods on the up-stroke. Since the ring side of thecylinder piston has less surface area than the cap side, thisconfiguration requires higher hydraulic pressure to develop the forcerequired to lift the rods.

In some embodiments, it may be desirable to suspend the hydrauliccylinder above the polished rod, using the retracting cylinder force tolift the rods from the well. Here again, the unit may be assembled atground level and pivotally erected into place.

The embodiments described herein also have a number of inherentadvantages over traditional hydraulic pumping units configured with wirerope sheaves. In the prior art, the wire rope often become a high wearitem due to the physical limitations of the size of the sheave. Whileconventional “walking beam” pumping units are configured with awire-rope bending radius of 70 inches or more, the prior art hydraulicunits are typically configured with rope sheaves with a bending radiusof 12 inches or less. This small diameter bending radius severelyreduces the life of the wire rope, thus increasing maintenance cost, andincreasing HSE risk due to frequency of wire rope failures. In contrast,to the rigidly mounted cylinders provided in the prior art, theembodiments described herein allow use of larger radius sheaves due tothe freely pivoting nature of the cylinder attachment to the wellstructure. In such a case, the angle of the cylinder with respect to thepolished rod would infinitely change as the cylinder passes between theretracted and extended position.

Other advantages are present with respect to multiple symmetricallyarranged hydraulic cylinders. Some hydraulic pumping units require twoor more specially designed hydraulic cylinders rigidly attached betweenupper and lower mounting plates, the illustrative embodiments mayutilize a single cylinder configuration. Fewer cylinders wouldinherently reflect lower initial cost, as well as lower futuremaintenance cost. Further still, regardless of the number of hydrauliccylinders, these embodiments may utilize low-cost, commodity typehydraulic cylinder configured with pin and clevis end connections. Thesecylinders are less expensive than custom-manufactured cylinders with aridged mounting base arrangement required in the prior art. In someembodiments, the cylinders may be rigidly attached to a verticalmounting beam at multiple points. Thus able to are better able to handlecolumn loading than the base-mounted cylinders found in the prior art.

Still other advantages are present with respect to rigid mountedcylinder configurations, whether that be single or multiple cylinderconfigurations. Rigid mounting of the cylinders with respect to thepolished rod requires a high degree of accuracy in manufacturing andinstallation so as to cause the lifting force to be aligned parallel andcongruent with the polished rod. Yet even with such care, there maystill be some slight misalignment. Such misalignment of rigidly mountedcylinders will inherently cause side loads and wear between the polishedrod and the stuffing box, and between the hydraulic cylinder rod and thecylinder rod bushings. The gimbaled mounting presented in thisillustrative embodiment overcome these deficiencies by providingpivoting and sliding degrees of freedom with respect to the wellheadmounting pins, thus allowing the lifting force to be always transmittedparallel to the polished rod.

The illustrative embodiments also overcome problems associated with lowpressure wellhead fixtures. In contrast to the bolted flanged facetubing holder utilized in high pressure wellheads, many low pressurewellheads utilize hammer union pack-off assemblies, thus lacking anyflat faced surfaces to which a base plate or cylinder mounting assemblycould be bolted or fastened. The illustrative embodiments overcome thatproblem by utilizing existing, symmetrically opposed wellhead pipingports as a means of attachment.

Hydraulic pumping units require a directional shifting valve tocyclically change the flow of oil and the directional motion of thehydraulic cylinders. An electric limit switch that senses the end ofeach stroke of the hydraulic cylinder may be used to shift a solenoidoperated hydraulic valve from one position to another. In an improvedsystem, mechanical controls linked to the movement of the hydrauliccylinder travel may be utilized to shift a valve spool from either the“up” or “down” position to the other. A spring detent mechanism isrequired to snap the valve from one position to the other so as toprevent the hydraulic directional valve from being stuck in a centerposition where no oil flows to the cylinder, preventing completion ofthe stroke. Unfortunately, sudden reversal of direction of the cylindercaused by the snap action of a spring detent mechanism on the valve cancause tensile or buckling fatigue and failure of the down-hole rodstring, in addition to accelerated wear from the shock loads on thehydraulic system. Soft shifting valve configurations can be used toreduce the acceleration and deceleration forces at the beginning and endof each stroke.

Hydraulic systems typically have an overall energy efficiency of between75%-85%. The loss of motive energy is translated into heating of thehydraulic fluid. This heat must be removed in order to prolong the lifeof the hydraulic components. Fan-powered heat exchangers may be used tocool the hydraulic fluid. In certain applications, particularly inshallow well pumping applications, the volume of fluid pumped from thewell may be sufficient to dissipate this excess heat. In this situation,a tube and shell heat exchanger may use the fluid pumped from the wellas a medium to dissipate the heat from the hydraulic fluid. Atemperature switch in the production fluid downstream of the heatexchanger could be used as a “pump-off” controller. Production fluidpumped from the well is normally at a constant temperature. As such, ahigher than normal temperature reading after the heat exchanger wouldsignal that the volume rate of the production fluid from the well isdecreasing, thus indicating the well is approaching a pumped-offcondition. This may trigger a control circuit to set the pumping unit toan idle or off state. Alternatively, a similar temperature monitoringcircuit could monitor the hydraulic fluid temperature for the samepurpose. In either case, a reset timer would be used to re-start thepump cycle at a pre-determined interval.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are included to illustrate certain aspects of thepresent disclosure, and should not be viewed as exclusive embodiments.The subject matter disclosed is capable of considerable modifications,alterations, combinations, and equivalents in form and function, withoutdeparting from the scope of this disclosure.

FIG. 1 illustrates a profile view of a hydraulic pumping unit in avertical operating position according to an illustrative embodiment, atraveling head assembly of the pump unit being near a top of the stroke;

FIG. 2A illustrates a profile view of the pumping unit of FIG. 1 in anassembly or well maintenance position;

FIG. 2B illustrates a top view of a mounting base attached to a wellheadaccording to an illustrative embodiment;

FIGS. 3A, 3B and 3C illustrate front, partial front and enlarged, andtop views of a traveling head assembly according to an illustrativeembodiment;

FIGS. 4A and 4B illustrate profile and side views of a pumping unitutilizing a roller chain to lift rods according to an illustrativeembodiments;

FIGS. 5A and 5B illustrate a profile view of a pumping unit utilizing amechanical spring and counter balance arrangements to reduce thesupplied energy to lift the rods according to an illustrativeembodiment;

FIGS. 6A and 6B illustrate profile views of a hydraulic pumping unit inan operating position according to an illustrative embodiment, thehydraulic pumping unit of FIG. 6A having a larger diameter sheave orsprocket than that of FIG. 6B;

FIGS. 7A and 7B illustrate profile and top views a shifting system for ahydraulic pumping unit;

FIG. 8 illustrates an enlarged exploded view of the shifting system ofFIG. 7A;

FIG. 9A illustrates an enlarge profile view of a plurality of stationarycomponents associated with the shifting system of FIG. 7A;

FIG. 9B illustrates a top view of a mounting frame used to mount theshifting system of FIG. 7A to a beam;

FIG. 9C illustrates an enlarged profile view of the shifting system ofFIG. 7A; and

FIG. 10 illustrates a schematic of a hydraulic circuit associated withthe shifting system of FIG. 7A.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following detailed description of the illustrative embodiments,reference is made to the accompanying drawings that form a part hereof.These embodiments are described in sufficient detail to enable thoseskilled in the art to practice the invention, and it is understood thatother embodiments may be utilized and that logical structural,mechanical, electrical, and chemical changes may be made withoutdeparting from the spirit or scope of the invention. To avoid detail notnecessary to enable those skilled in the art to practice the embodimentsdescribed herein, the description may omit certain information known tothose skilled in the art. The following detailed description is,therefore, not to be taken in a limiting sense, and the scope of theillustrative embodiments is defined only by the appended claims.

Unless otherwise specified, any use of any form of the terms “connect,”“engage,” “couple,” “attach,” or any other term describing aninteraction between elements is not meant to limit the interaction todirect interaction between the elements and may also include indirectinteraction between the elements described. In the following discussionand in the claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to”. Unless otherwise indicated, as used throughout thisdocument, “or” does not require mutual exclusivity.

As used herein, the phrases “hydraulically coupled,” “hydraulicallyconnected,” “in hydraulic communication,” “fluidly coupled,” “fluidlyconnected,” and “in fluid communication” refer to a form of coupling,connection, or communication related to fluids, and the correspondingflows or pressures associated with these fluids. In some embodiments, ahydraulic coupling, connection, or communication between two componentsdescribes components that are associated in such a way that fluidpressure may be transmitted between or among the components. Referenceto a fluid coupling, connection, or communication between two componentsdescribes components that are associated in such a way that a fluid canflow between or among the components. Hydraulically coupled, connected,or communicating components may include certain arrangements where fluiddoes not flow between the components, but fluid pressure may nonethelessbe transmitted such as via a diaphragm or piston.

FIG. 1 shows one embodiment of the hydraulic pumping unit 10 configuredin the operating position. The base 11 is pivotally attached to a rigidpart of the well structure 12. Shown here, the base is pivoting on lugs13 that are installed in unused ports of the wellhead 14. These lugs 13typically screw into wellheads 14 with 2 inch pipe threads. Thesethreads can easily support the reactive force generated by the hydraulicpumping unit in most shallow well applications. Symmetrically opposedports in the tubing head 9 may be used as well. The hydraulic pumpingunit 10 may also be supported by symmetrically opposed pipe nipplescarrying flow products from the well. For added strength, these nipplescan be thick-wall Schedule 80 or stronger pipe. Alternatively, similartypes of lugs can be welded or fastened to the well casing or a groundmounted structure so as to provide an alternate support to the pivotingbase. The mounting lugs may be positioned close to the ground so as tofacilitate easy access for installation and maintenance.

An upright beam 15 is rigidly attached to the base 11. In the operatingposition, the beam is horizontally offset, but approximately parallelwith the polished rod 16. Typically, a standard 6 or 8 inch wide-flangeI-beam would be used for most shallow well applications. In someembodiments, the beam 15 merely acts as a guide for the traveling headassembly 17 and is not subject to column loading. Light strengthmaterial such as aluminum or fiberglass composite can be used instead ofsteel.

One or more hydraulic cylinders 18 are connected between the base 11 andthe head assembly 17. The length of the hydraulic cylinder 18 determinesthe stroke of the pumping unit. The polished rod load determines therequired diameter of the hydraulic cylinder and the system pressure. Insome long stroke applications, limitations on the buckling strength ofthe hydraulic cylinder rod 19 may be design criteria. Typically, asingle 4 inch diameter hydraulic cylinder would meet the design criteriafor most shallow well applications. Alternately, multiple cylinders canbe symmetrically arranged about the beam 15 to provide additional liftcapacity.

FIG. 2A shows a configuration of the hydraulic pumping unit in theout-of-service position. When well-work is required, the head assembly17 may be detached from the polished rod 16, and the pumping unit 10allowed to pivot out of the way of any down-hole operations. Initialassembly and future periodic maintenance of the pumping unit wouldsimilarly occur in this lay-down position as well. The unit would thenbe rotated back into the operating position using either man-power, or asimple winching device.

FIG. 2B shows the top view of the base 11 pivoting on lugs 13. Pivotbushing 19 can be of any suitable bearing material, such as brass orsteel. Alternatively, other configurations may employ roller or ballbearings to reduce wear. While the base 11 is designed to pivot aboutone axis, a small amount of lateral movement perpendicular to the pivotaxis may be provided so as to also accommodate small misalignment inthat axis as well. The movement of the base 11 about the non-pivotingaxis may be through sliding of the pivot bushing 19 along the pipe lugs13, or through a gimbaled or linear bearing assembly. As such, theentire pumping unit has multiple degrees of freedom.

FIG. 3A shows the profile view of the head assembly of one configurationof the pumping unit. This head assembly 17 is used to transmit the forceof the hydraulic cylinder rod 19 to the polished rod 16. The head 17assembly travels along a track formed by the beam 15. Typically, rollers20 would be used to reduce friction as the head moves up and down alongthe beam 15. The rollers 20 can be flanged so as to keep aligned withthe edge of the beam 15. Alternate configurations may include rollersthat roll against both front and back flanges of beam 15. The rollers 20can be steel, aluminum, polyurethane, nylon, or any other suitablematerial. To minimize maintenance cost, replaceable rollers arepreferably designed to wear ahead of the beam 15.

FIG. 3B shows the detail of the attachment of the polished rod 16 to thehead assembly 17. In one configuration, a spherical bearing 21 is usedbetween the rod clamp 22 and the base plate 23 to allow for anymisalignment in the travel of the head assembly 17. The base plate 23 isconfigured with a bearing holder 24 such that bearings 21 may be easilyreplaced if necessary. Alternate head bearing assembly configurationsmay also be employed to similarly provide freedom of movement in eitherone or two axis. The use of this head bearing assembly, in combinationwith the pivot and slid bearing of the base, provide the geometrynecessary to eliminate side loads while lifting the polished rod 16.

FIG. 3C illustrates the top view of the head assembly 17. In theillustrated configuration, normally the weight of the down-hole rodstring acting on the polished rod 16 keeps the rollers in contact withthe beam 15. At times when downward force is not present, such as duringinstallation or when the down hole pump becomes “stacked-out”, guide 25and back-plate 26 serve to keep the head assembly 17 from rotatingbeyond the flange face of the rollers 27. Other roller or guideconfigurations could provide similar function to keep the head assembly17 aligned with the beam 15 at all times.

FIG. 4A shows an alternate configuration to the track mounted travelinghead arrangement previously described. Here, one or more hydrauliccylinders 28 are rigidly attached to the beam 15. Freely turningsprockets 29 or sheaves are attached to the traveling portion of thecylinder 28. Because the entire hydraulic pumping unit 30 can pivot inrelation to the well structure, the diameter of the sheave or sprocket29 has no bearing on the angle of force applied to the polished rod 16.In one embodiment, roller chain 31 is connected to the bridle 32 to liftthe polished rod 16. The other end of the roller chain is “dead-lined”33 and fastened to the beam 15.

FIG. 4B reflects the front view of sprocket or sheave head assembly. Thevertical spacing of the rod clamp 34 to the polished rod 16 is achievedthrough adjusting the point of attachment of the cylinder mountingbracket 8 to the beam 15, and through changing the length of the rollerchain 31 attached to the bridle 32. In the event of a failure of asingle roller chain or wire rope, load sensing circuits can be used toshut off the supply of hydraulic fluid so as to prevent asymmetricalforces to the pumping unit.

FIG. 5A illustrates the counter-balancing of the off-center mass of thepumping unit. For safety reasons, it is important to provide somerestraint of the pumping unit 34, in the event the polished rod 16fails, or some other instance that could allow the pumping unit tounexpectedly fall away from the upright position. A spring 35 may beused to counteract and balance the overhung load of the pumping unitabout the pivot point 37. This counter-balance force may also be in theform of a counter weight 36 on the opposite side of the pivot point 37.Alternately, if minimal pivot movement is desired, a ridged or semiridged attachment may be employed in place of the spring 35 to restrainthe hydraulic pumping unit 34 to the well structure 12.

Energy efficient hydraulic circuits may be employed with the hydraulicpumping units described herein. A charge of compressed gas such asnitrogen may be used in the form of an accumulator to balance the deadweight of the rod string. Another form of energy savings applicable inthis and other hydraulic pumping units is to use one or more coilsprings 38 to serve the same balancing function as the hydraulicaccumulator. In one embodiment, the springs would be sized such that theaverage hydraulic energy necessary to lift the polished rod 16 on theupstroke is identical, but opposite that required to retract the pump onthe down stroke. For example, if the polished rod load is 10,000 lbs.during the upstroke, and the dead weight of the rod string in the wellis 5,000 lbs., the springs 38 would be sized to provide an averagelifting force of 7,500 lbs. This simple example ignores the differencein force developed on the ring side and cap side of the hydrauliccylinder. In practice, the ideal mechanical spring configuration wouldattempt to make the hydraulic horsepower equal on both the up stroke andthe down stroke.

FIG. 5B illustrates how multiple springs 38 could be installed toprovide a lifting force to the head assembly 17 in order to more evenlydistribute the duty cycle of the hydraulic system. Springs may bestacked, or concentrically arranged to provide the desired force. Centercore 39 provides lateral support for the springs. Die springs are ableto provide the millions of cycles necessary for this type ofapplication.

FIG. 6A illustrates the significant pivot motion of a hydraulic pumpingunit throughout the stroke cycle when using a large diameter sheave orsprocket. Large diameter sheaves provide longer life for wire rope. FIG.6B illustrates a hydraulic pumping unit with a sheave or sprocket sizedto minimize pivot motion.

In contrast to the prior art, a simplified shifting and control systemis now presented. As illustrated in FIG. 7A, a control rod 40 isattached through linkage 50 to the head assembly 47 so as to be indexedwith the travel of hydraulic cylinder 28. The control rod 40 is used totransmit the mechanical motion necessary to shift the directionalcontrol valve 55 from one position to the other. Adjustable positionstops 44 attached to control rod 40, acting on shift springs 43, areused to set the actual point of directional change in relation to thestroke of cylinder 28. In the up-stroke valve position, the pressurizedhydraulic line 57 delivering fluid from the hydraulic pump 58 and isfluidly coupled to the lower end of the hydraulic cylinder 28, causingthe cylinder to extend. In the down-stroke valve position, the lower endof the hydraulic cylinder 28 is fluidly connected to the hydraulic fluidreturn path 59, thus returning hydraulic fluid to the reservoir tank andallowing the cylinder to retract. The speed of the up-stroke cylindermovement is dependent on the volume rate of the hydraulic pump 58.Independently, the speed of the falling cylinder may be controlled witha needle valve 60, an over-balance valve, or other hydraulic controlmechanisms.

FIG. 7B illustrates a top view through cross section AA of FIG. 7A. Asillustrated here, control rod 40 is connected to head assembly 47through linkage 50. The control rod 40 may be positioned to run alongany point of beam 15, here illustrated as traveling within the flangesof an “I” beam.

FIG. 8 illustrates an exploded view of one embodiment showing variouscomponents of a spring detent snap-action shift mechanism. Asillustrated, control rod 40 travels up and down through shift tube 62,which in turn is free to travel within guide tube 63. As the travel ofcontrol rod 40 begins to approach the point representing the end of thehydraulic cylinder stroke, control rod stop 44 begins to force the shiftspring 43 against the shift tube 62. Shift tube 62 in turn appliesforce, either directly or indirectly to valve spool 41. To minimize wearon the valve spool resulting from the millions of cycles of repeatedshifting, and in recognition of potential slight miss-alignment of theshifting components, a non-ridged coupling of the shifting tube 62 tothe valve spool 41 may be preferred.

Valve spool 41 is held in place by spring detent assembly 42,illustrated here in prospective view showing only one half of thesymmetrically arranged roller bearings that will seat in either theupper detent seat or lower detent seat, each seat corresponding to arespective valve position. The travel of control rod 40 causes shiftspring 43 to be compressed until such time there is sufficient force tosuddenly unseat detent mechanism. Once unseated, compressed shift spring43 begins to force valve spool 41 from the present position to theother. Instead of snapping quickly to the alternate valve position,velocity control devices 45, coupled to valve spool 41, dampens thestored energy of the shifting spring 43, thus allowing the valve spoolto travel slowly and smoothly from one position to the other. Velocitycontrol device 45 may be an adjustable, variable orifice shock absorber,commercially available to control the velocity of an object being actedupon with an applied force. In a preferred embodiment, the amount oftime involved in the shifting process is between 1-5 seconds.

FIG. 9A illustrates one embodiment of a preferred mounting arrangementof the various components of the hydraulic shifting system. In thisarrangement, stationary components including directional control valve55, guide tube 63, and velocity control device 45 are rigidly attachedto mounting frame 49, which in turn mounts to a ridged structure of thehydraulic pump beam. As illustrated, hydraulic directional control valve55 is mounted to the front face of mounting plate 52, while guide tube63 is mounted behind. FIG. 9B illustrates the mounting frame 49 attachedto the beam 15. Set screws 53 may be used to provide adjustableattachment of mounting frame 49. As illustrated, mounting frame 49 isshown mounted within the flanges of an “I”-beam. Control rod 40 isillustrated passing behind mounting frame 49. FIG. 9C illustrates theoperation of the shift control mechanism with the addition of a controllever 54 for manual activation of directional control valve 55.

While one embodiment of “soft” mechanical shifting of a hydraulicdirectional control valve has been described herein, it should be notedthat other types of mechanical hydraulic control could provideadditional beneficial results. For instance, a valve spool profile canconfigured so as to provide taper so as to create a variable Cv factoras it travels from one position to the next. In such a case, both valvespool position within the valve body and valve spool geometry canprovide non-linear velocity control as the spool changes from one flowpath to the other.

In yet another embodiment, two separate valves can be linked to themovement of the hydraulic cylinder stroke such that one valve providesvariable flow control, and another separate valve provides directionalcontrol. For instance, as the hydraulic cylinder approaches the end of astroke, a flow control valve begins to progressively reduce the volumerate of fluid flowing to the directional shifting valve. Based on thedesign of the control valve, the reduction in flow can be in direct orvariable relationship to the position of the cylinder with respect tothe desired reversal point. An adjustable stop on the flow control valveis used to set a minimum fluid volume rate so as to allow the cylinderto “creep” slowly to the point where direction is reversed. Similar tothe function as previously described, a detent used in combination withshifting springs would be used to shift a separate directional controlvalve. Upon shifting direction, the control valve would progressivelyopen from “creep” for full open position.

FIG. 10 illustrates one embodiment of a simplified hydraulic circuitusing a minimal number of hydraulic component. Electric motor or otherprime mover 69 is connected to drive hydraulic pump 58. Relief valve 71is provided, but provides no operational function other than forover-pressure safety issues. Two position directional control valve 55is positioned for either extending the cylinder, or allowing thecylinder to retract. The valve is illustrated in the extend-cylinderposition. In this configuration, fluid from reservoir tank 80 flows intothe suction of the hydraulic pump 58. It is noted that the reservoirtank 80 can be either open to atmosphere, or a nitrogen chargedaccumulator so as to assist with lifting the dead weight of the downholerod string. In either case, hydraulic fluid is discharged from the pump58 and flows through the by-pass of needle valve 74 into the base ofhydraulic cylinder 75, causing the cylinder to extend. For theretract-cylinder phase of the cycle, directional control valve 55 shiftsto a second position allowing hydraulic fluid from the extended cylinderto return through needle valve 74, through valve 55, then ultimately toreservoir tank or accumulator 80. While directional control valve 55 isin this retract position, hydraulic fluid from the pump is similarlydirected to the same common return path as the fluid from the retractinghydraulic cylinder. As such, the motor 69 and pump 58 are unloaded whenhydraulic pressure isn't needed, thus eliminating the need for expensivepressure compensated, variable displacement pumps, as well aseliminating repeated starting and stopping the motor pump combination.

The heat exchanger 76, necessary to maintain oil temperature withinoperating limits, may be of tube and shell construction. In contract toconventional air cooled-radiator type oil coolers, tube-and shell oilcoolers can be designed to handle the high operating pressure of thehydraulic system. In one embodiment, fluid pumped from the well may beused as the coolant. As illustrated, heat exchanger 76 is located justprior to the accumulator or tank 80. It is noted that the heat exchangercan be located in most any flow path in the system.

While the embodiments described herein refer to the term “hydraulic”when describing the motive fluid used to raise and lower the cylinders,it should be noted that any type of fluid or mechanical energy could besimilarly employed to achieve the same results, including pneumaticsources of energy. For example, an internal combustion engine may belocated at or near the location of the pumping unit. In such a case,waste heat from the engine may be converted into steam, either alone orcombined with additional input energy. This steam could be used toprovide the fluid power necessary to raise and lower the pumping unit.Alternate forms of mechanical linear actuators may also be used toprovide the lifting force described herein as that produced by ahydraulic cylinder.

It should be apparent from the foregoing that an invention havingsignificant advantages has been provided. While the invention is shownin only a few of its forms, it is not limited to only these embodimentsbut is susceptible to various changes and modifications withoutdeparting from the spirit thereof.

What is claimed is:
 1. A reciprocating well pumping unit comprising: afluid-driven cylinder adapted to be coupled to a reciprocating rodassembly, the cylinder having a lifting rod capable of traveling betweenan extended position and a retracted position; a valve capable ofcontrolling fluid delivery to the cylinder, the valve having a valvespool capable of being positioned in a first position or a secondposition, the valve in the first position causing fluid flow that allowsthe lifting rod to extend, the valve in the second position causingfluid flow that allows the lifting rod to retract; a detent assemblyproviding resistance to movement of the valve spool between the secondposition and the first position; a control rod coupled to the liftingrod such that movement of the lifting rod in a first direction resultsin movement of the control rod in the first direction; a rod stopcoupled to the control rod; and a spring carried on the control rod andcapable of engagement with the rod stop, the spring capable of storingenergy as the control rod is moved in the first direction and the rodstop engages the spring, the spring engaging the valve spool as energyis stored in the spring to urge the valve spool toward the firstposition.
 2. The reciprocating well pumping unit of claim 1, whereinwhen a threshold amount of energy is stored in the spring, the detentassembly permits the movement of the valve spool into the firstposition.
 3. The reciprocating well pumping unit of claim 1 furthercomprising: a dampener operably associated with the valve spool or thecontrol rod to control a velocity of the valve spool or the control rod.4. The reciprocating well pumping unit of claim 1, wherein the detentassembly further comprises: a detent seat having at least one detentstop; at least one detent arm supporting a detent bearing and beingpivotally movable between an engaged position in which the detentbearing engages the at least one detent stop and a disengaged positionin which the detent bearing becomes disengaged from the at least onedetent stop; a detent spring to bias the at least one detent arm towardthe engaged position.
 5. The reciprocating well pumping unit of claim 4,wherein the at least one detent stop is a recess or a projectiondisposed on the detent seat.
 6. The reciprocating well pumping unit ofclaim 1 further comprising: a second rod stop coupled to the controlrod; a second spring carried on the control rod and capable ofengagement with the second rod stop, the second spring capable ofstoring energy as the control rod is moved in the second direction andthe second rod stop engages the second spring, the second springengaging the valve spool as energy is stored in the second spring tourge the valve spool toward the second position; wherein the detentassembly provides resistance to movement of the valve spool between thefirst position and the second position.
 7. The reciprocating wellpumping unit of claim 6 further comprising: at least one dampeneroperably associated with the valve spool or the control rod to control avelocity of the valve spool or the control rod.
 8. The reciprocatingwell pumping unit of claim 6, wherein the detent assembly furthercomprises: a detent seat having a first detent stop and a second detentstop; at least one detent arm supporting a detent bearing and beingpivotally movable between an engaged position in which the detentbearing engages one of the first and second detent stops and adisengaged position in which the detent bearing becomes disengaged fromthe one of the first and second detent stops; a detent spring to biasthe at least one detent arm toward the engaged position.
 9. Areciprocating well pumping unit comprising: a beam assembly pivotallyattached to an anchor member that is fixed relative to a well; afluid-driven cylinder having a cylinder housing and a cylinder rod, thecylinder rod capable of extending from or retracting into the cylinderhousing; and a traveling assembly adapted to be coupled to areciprocating rod assembly extending into the well, the travelingassembly coupled to one of the cylinder rod and the cylinder housing;wherein the beam assembly is capable of pivoting during operation of thefluid-driven cylinder.
 10. The reciprocating well pumping unit of claim9, wherein an axis of rotation about which the beam assembly is capableof pivoting intersects a longitudinal axis of the reciprocating rodassembly.
 11. The reciprocating well pumping unit of claim 10, whereinthe beam assembly is capable of lateral movement along the axis ofrotation during operation of the fluid-driven cylinder.
 12. Thereciprocating well pumping unit of claim 9, wherein the anchor member isa wellhead of the well and the pivotal coupling between beam assemblyand the wellhead is located at a fluid port of the wellhead.
 13. Thereciprocating well pumping unit of claim 12, wherein a conduit isfluidly connected to the fluid port to allow fluid communication betweenthe conduit and a wellbore of the well.
 14. The reciprocating wellpumping unit of claim 9, further comprising a fluid source fluidlyconnected to the cylinder housing.
 15. The reciprocating well pumpingunit of claim 9, wherein the beam assembly further comprises: a beam;and a base coupled to the beam.
 16. The reciprocating well pumping unitof claim 15, wherein the beam is coupled to another of the cylinder rodand the cylinder housing.
 17. The reciprocating well pumping unit ofclaim 15, wherein the base is coupled to another of the cylinder rod andthe cylinder housing.
 18. The reciprocating well pumping unit of claim15, wherein the traveling assembly further comprises: a traveling head;a plurality of rollers rotatingly coupled to the traveling head, theplurality of rollers engaging the beam to allow the traveling head totravel along the beam in a direction substantially parallel to themovement of the reciprocating rod assembly.
 19. The reciprocating wellpumping unit of claim 15, wherein the beam is an I-beam having a firstflange and a second flange joined by a transverse member.
 20. Thereciprocating well pumping unit of claim 19, wherein the travelingassembly further comprises: a traveling head; and a plurality of rollersrotatingly coupled to the traveling head, the plurality of rollersengaging the beam to allow the traveling head to travel along the beam;wherein a first and a second of the plurality of rollers are eachpositioned on an opposing side of the first flange of the beam.
 21. Thereciprocating well pumping unit of claim 9, wherein the travelingassembly further comprises: a rotating member rotatingly coupled to theone of the cylinder rod and the cylinder housing; a flexible linkagehaving a first end coupled to the beam assembly and a second end coupledto the reciprocating rod assembly; wherein the rotating member engagesthe flexible linkage such that extension or retraction of the cylinderrod moves the rotating member thereby resulting in the lifting orlowering of the reciprocating rod assembly.
 22. The reciprocating wellpumping unit of claim 21, wherein: the rotating member is a sheave or asprocket; and the flexible linkage is a cable, a rope, or a chain. 23.The reciprocating well pumping unit of claim 9 further comprising: acounter weight coupled to the beam assembly to counteract at least aportion of the weight of the beam assembly about the pivotal coupling ofthe beam assembly.
 24. A reciprocating well pumping unit comprising: afluid-driven cylinder having a cylinder housing and a cylinder rod, thecylinder rod capable of extending from or retracting into the cylinderhousing, one of the cylinder rod and the cylinder housing being coupledto a reciprocating rod assembly extending into a well, another of thecylinder rod and the cylinder housing being fixed relative to thereciprocating rod assembly; a heat exchanger having a first fluidpathway fluidly connected to the fluid-driven cylinder, the first fluidpathway receiving a first fluid used to drive the cylinder rod, the heatexchanger having a second fluid pathway to receive a production fluidfrom the well; and a temperature determination unit operably associatedwith either the first or the second fluid pathway downstream of the heatexchanger to determine a temperature of the production fluid followingexit of the production fluid from the heat exchanger; wherein the flowof the first fluid to the fluid-driven cylinder is adjusted in responseto the temperature determination.
 25. The reciprocating well pumpingunit of claim 24, wherein the temperature determination unit furthercomprises a temperature sensor.
 26. The reciprocating well pumping unitof claim 24, wherein the temperature determination unit furthercomprises a temperature switch.
 27. The reciprocating well pumping unitof claim 24, wherein the temperature switch is configured to turn off apumping circuit, thereby ceasing reciprocation of the reciprocating rodassembly.
 28. The reciprocating well pumping unit of claim 24, whereinwhen the temperature is greater than or equal to a thresholdtemperature, an operator is notified that pumping operations should beceased.
 29. The reciprocating well pumping unit of claim 24, wherein theone of the cylinder rod and the cylinder housing is coupled to thereciprocating rod assembly by a traveling assembly.
 30. Thereciprocating well pumping unit of claim 24, wherein the flow adjustmentof the first fluid further comprises ceasing delivery of the first fluidto the fluid-driven cylinder.